1. Field of the Invention
The present invention relates to an optical modulator, and more particularly, to an optical modulator including a thin plate made of a material having an electrooptic effect and having a thickness of 20 μm or less.
2. Related Art Statement
Conventionally, in an optical communication field or an optical measurement field, a waveguide type optical modulator in which an optical waveguide or a modulation electrode is formed on a substrate having an electrooptic effect is much used.
Particularly, as multimedia has been developed, the amount of communication tends to increase and thus wider bandwidth of an optical modulation frequency needs to be realized. As a realizing means thereof, an external modulation method using a LN modulator or the like has been diversified. However, in order to realize the wider bandwidth of the LN modulator, velocity matching between a light wave and a microwave which is a modulation signal and reduction of a driving voltage need to be realized.
As a means for solving the above-described problems, a technique for reducing the thickness of a substrate such that the velocity matching condition between the microwave and the light wave is satisfied and the driving voltage is reduced is conventionally known.
In Patent Document 1 or 2, an optical waveguide and a modulation electrode are mounted on a thin substrate (hereinafter, referred to as “first substrate”) having a thickness of 30 μm or less and another substrate (hereinafter, referred to as “second substrate”) having dielectric constant lower than that of the first substrate is bonded to the first substrate, such that an effective refractive index of the microwave is lowered, the velocity matching between the microwave and the light wave is realized and mechanical strength of the substrate is maintained.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. S64-18121
[Patent Document 2] Japanese Unexamined Patent Application Publication No. 2003-215519
Meanwhile, in the waveguide type optical modulator, an optical waveguide having an optical junction portion in which a plurality of optical waveguide portions are joined together is formed, similar to a Mach-Zehnder type optical waveguide. Radiation light is radiated from the optical junction portion to the outside of the optical waveguide according to the condition of the joined light wave. As disclosed in Patent Document 3, the operation state of the optical modulator is monitored using the radiation light.
[Patent Document 3] Japanese Unexamined Patent Application Publication No. 2001-281507
In a case of a thick plate structure (100 μm or more), the radiation light is radiated from the surface of the substrate to the inside of the substrate, as shown in FIG. 1. Thus, characteristic deterioration of the modulator due to the radiation light may be commercially ignored. In Patent Document 4, an output waveguide is bent to adjust the positions of the propagation light and the radiation light of the waveguide, thereby separating the light. Since Patent Document 4 is premised on lens coupling, there is a problem that the propagation light and the radiation light are mixed. However, in a “pigtail type optical modulator” in which an optical fiber is directly connected to the optical modulator, since the propagation light 7 and the radiation light 8 are separated from each other by 100 μm or more as shown in FIG. 1, the radiation light is not inputted into the fiber.
[Patent Document 4] Japanese Unexamined Patent Application Publication No. H8-194195
However, in the above-described thin plate, the thin plate functions as a slab waveguide. Thus, as shown in FIG. 3, the radiation light forms a radiation light spot 8 which is spread in a lateral direction (direction parallel to the surface of the substrate) in the end portion of the thin plate. Thus, a light spot (propagation spot) 7 of the propagation light which propagates in the optical waveguide and a portion of the radiation light spot 8 are close or overlapped to each other, and thus a portion of the radiation light is inputted into an optical fiber when the propagation light is introduced into the optical fiber. As a result, an extinction ratio of the optical modulator deteriorates or misalignment at the time of connecting the fiber is apt to be caused due to overlapping between the radiation light spot and the propagation light spot.
FIG. 2A is a view showing an optical modulator when viewed from the top, and, for simplification of description, an electrode and a buffer layer are omitted. Reference numeral 1 denotes a thin plate, 4 denotes a Mach-Zehnder type optical waveguide, 5 denotes an output optical waveguide portion of the waveguide, 6 denotes input light, and 8 denotes radiation light radiated from an optical junction portion of the optical waveguide. FIG. 2B shows the end portion of the output side of the optical modulator. Reference numeral 7 denotes a propagation light spot of propagation light which propagates in the output optical waveguide portion 5, 8 denotes a radiation light spot, 2 denotes an adhesive layer, and 3 denotes a reinforcement plate for reinforcing the thin plate 1. FIG. 3 shows an intensity distribution of the light 7 from the output optical waveguide and the radiation light 8 when the thickness of the thin plate is 10 μm.